Sulfonic acids of aromatic polyisocyanates are known and have been described, for example, in German Offenlegungsschriften No. 2,227,111 and 2,359,615 and U.S. Pat. No. 3,826,769. They are obtained by simply reacting the corresponding aromatic polyisocyanate with a sulfonating agent such as sulfur trioxide, an adduct of sulfur trioxide, oleum, chlorosulfonic acid or sulfuric acid.
The product obtained may be a solid, resinous, or pulverulent sulfonation product or a solution or dispersion of a sulfonated isocyanate in unchanged starting material, depending on the isocyanate used and the degree of sulfonation.
The preparation of polyurethanes or polyurethane ureas based on polyisocyanates containing sulfonic acid groups has been described, for example, in U.S. Pat. No. 3,826,769 and in German Offenlegungsschrift No. 2,359,611. Polyurethanes and polyureas which have been modified in this way with sulfonic acid or sulfonate groups frequently have a very hydrophilic character, and for this reason the proportion of sulfonic acid groups is generally kept as low as possible. Partially sulfonated polyisocyanates and solutions of isocyanato-sulfonic acids in polyisocyanates are of special importance for this reason. The main advantage of sulfonated polyisocyanates is due to the high polarity of sulfonic acid groups. This gives rise to marked interactions or forces of adhesion in relation to polar media and substrates such as water, and moist and polar surfaces particularly those of an inorganic nature.
Chemically uniform or persulfonated isocyanatosulfonic acids, on the other hand, have high melting points and a low degree of solubility in organic media. Also, they are extremely sensitive to water and atmospheric moisture. These characteristics make the products difficult to process. The highly acid character is also frequently a disadvantage. Discoloration and premature degradation of polyurethanes produced from these substances make the products difficult to use in practice. Although these phenomena can be prevented by suitable neutralization methods, these increase the hydrophilic character of the products.
There is therefore a demand for polyisocyanates which would have a polar character (with its attendant advantages such as a low vapor pressure and improved adhesiveness) and would, at the same time, be predominantly hydrophobic.
There is also a demand for readily available aromatic polyisocyanates from which could be synthesized oligomers and polymers which give rise to toxicologically harmless products on hydrolytic degradation.